(1) Field of the Invention
The present invention relates to a shift apparatus which shifts a clutch into forward, neutral and reverse and is provided in an outer drive apparatus of an inboard-outboard drive.
(2) Description of the Related Art
An inboard-outboard drive (a.k.a., stern drive) is a form of marine propulsion in which a main engine ((a.k.a., inboard motor) is provided inside the hull and an outer drive apparatus is provided on the outside of the hull. The outer drive apparatus is a propulsion unit which integrally incorporates a reverse reduction gear, clutch mechanism, steering mechanism, propellers and is attached to a transom portion.
In known inboard-outboard drives, a shift mechanism for switching the clutch mechanism into forward, neutral or reverse generally employs a mechanical mechanism in which a shift operation lever in the vessel and a clutch lever in the outer drive apparatus are connected by a cable such as a wire (for example, Japanese Unexamined Patent Publication No. 4-254289).
The case where an hydraulic clutch is employed in an inboard-outboard drive of this type will be described with reference to FIG. 7 as an example of the oil-hydraulic circuit: a pressure oil discharged from a gear pump 30 which receives a drive force from an output shaft of a main engine is transferred to a clutch 8 through a forward/reverse directional control valve 50. The forward/reverse directional control valve 50 is a mechanically operated valve and switched by a shifter 51. This shifter 51 is operated by a clutch lever (not shown), and the clutch lever is connected with a shift operation lever (not shown) in the vessel by a wire cable. The oil-hydraulic circuit is provided with a relief valve 56 having a slowly engaging function to reduce the impact of rapid engagement on the clutch 8. The relief valve 56 is provided with two spring bearings 56a, 56b which are in the form of hydraulic pistons capable of compressing a pressure regulating spring 56s and disposed serially in a cylinder 56c. The relief valve 56 is additionally provided with a pressure-regulating circuit formed by connecting throttling passages split from a forward output port and reverse output port of the forward/reverse directional control valve 50 to oil chambers 56d, 56e of the spring bearings 56a, 56b, respectively. When the forward/reverse directional control valve 50 is in the neutral position (as in FIG. 7), the spring bearings 56a, 56b are in the most retracted positions due to a biasing force of the pressure-regulating spring 56s and the relief valve 56 operates as a relief valve having a low setting pressure. When the forward/reverse directional control valve 50 is switched to forward or reverse, the spring bearing 56a or 56b moves to compress the pressure-regulating spring 56s with a time delay. When the setting pressure of the relief valve 56 gradually increases and the spring bearing 56a or 56b reaches a specified stroke, the maximum pressure of hydraulic operating fluid for the clutch is obtained. Thus, the pressure of the hydraulic operating fluid for the clutch is gradually increased. The drive force of the main engine is transmitted in this order: the clutch 8, the forward side gear 5 or reverse side gear 6 which is engaged with the clutch 8, a bevel gear 24, a drive shaft 23, a bevel gear 7, a propeller shaft 25, and a propeller 12.
As the clutch mechanism of the outer drive apparatus, friction clutches such as a multiple disc clutch (for example, Japanese Unexamined Patent Publication No. 4-254289) or a cone clutch (for example, Japanese Unexamined Patent Publication No. 3-10990) or claw clutches such as a dog clutch are employed.
Although more and more control systems for inboard-outboard drives are electronically controlled in recent years, mechanical mechanisms using a wire cable are still employed for shift mechanisms. When shift switching signals need to be electrical signals, the wire cable is operated by an actuator such as an electric motor installed in the vessel and controlled by electrical signals from a controller in the vessel.
However, in installing the actuator, there have been problems with the numerous man hours required to couple the main engine inside the vessel and the outer drive apparatus outside the vessel, such as for example, positioning of the wire cable for the forward, neutral and reverse positions.
Furthermore, since the wire cable have minimum bend radius and the wire cables themselves slide during switch operation, passage for the wire cable inside the outer drive apparatus is limited. Requirements for the passage in the outer drive apparatus, which has no extra space, have thus been considerably difficult to fulfill.